Translations by Andriy Korchak

Only add software repositories from sources that you trust. Third-party software repositories are not checked for security by Ubuntu members, so could contain software which is harmful to your computer.

Open <menuchoice><guimenu>System</guimenu><guimenuitem>Administration</guimenuitem><guimenuitem>Software Sources</guimenuitem></menuchoice> and select <guilabel>Third Party Software</guilabel>.

If the application that you want to uninstall is not available in Add/Remove Applications, use Synaptic (<menuchoice><guimenu>System</guimenu><guimenuitem>Administration</guimenuitem><guimenuitem>Synaptic Package Manager</guimenuitem></menuchoice>) to remove it instead.

Find the application that you want to remove by finding it in the list or by searching for it.

Click <guibutton>Apply Changes</guibutton>, then click <guibutton>Apply</guibutton> to remove the applications. Enter your password if prompted.

Uncheck the box next to the application that you want to remove. Do this for all of the applications you want to remove.

Synaptic can be used to manage advanced software packages (such as server applications) which Add/Remove Applications does not manage.

Check the box next to the application that you want to install. Do this for all of the applications that you want to install.

The package manager downloads applications from a <emphasis>software repository</emphasis>, which is a location on the Internet which stores a collection of applications. These applications come bundled in <emphasis>packages</emphasis>, which contain all of the information needed for installation. You can download packages yourself, using your web browser, if you like, but it is generally much more convenient to let the package manager handle this for you.

Some packages depend on other packages being installed in order to work. For example, a word processing package may require a printing package to be installed. The package manager automatically installs these <emphasis>dependencies</emphasis> for you.

By default, only applications from the official Ubuntu software repositories are available in your package manager. If you cannot find the application that you want in the default repositories, you can add other (<quote>third-party</quote>) repositories and install it from there.

Click <menuchoice><guimenu>System</guimenu><guimenuitem>Administration</guimenuitem><guimenuitem>Synaptic Package Manager</guimenuitem></menuchoice>. Enter your password if prompted.

Click <guibutton>Search</guibutton> to search for an application, or click <guibutton>Sections</guibutton> and look through the categories to find one.

Right-click the application that you want to install and select <guilabel>Mark for Installation</guilabel>.

If you are asked if you would like to mark additional changes, click <guibutton>Mark</guibutton>.

Click <guibutton>Apply</guibutton>, and then click <guibutton>Apply</guibutton> in the window that appears. The applications that you chose will be downloaded and installed.

You can download and install applications from websites. These applications are contained in Debian (.deb) packages. To install a Debian package:

Double-click the package. It will be opened in the Package Installer.

The packages have not been checked for security by Ubuntu members, and could contain software which harms your computer. You should only download single packages from websites that you trust.

The package may require some extra software to run, which cannot be installed automatically. You will have to find and install this software yourself.

Clicking a link on a web page

Some web pages have links which install applications when you click them. These are known as <quote>apt:</quote> links. After clicking the link, you will be asked if you would like to install additional software. Click <guibutton>Install</guibutton> to start the installation process.

The application can only be downloaded and installed if it is available in a software repository which is currently enabled on your computer. This means that websites cannot trick you into installing software which is potentially harmful to your computer.

Some applications depend on others being installed in order to work properly. If you try to remove an application which is required by another application, both of those applications will be removed. You will be asked to confirm that this is what you want to happen before the applications are removed.

Adding a software repository

Software is available from third-party sources, as well as from the default Ubuntu software repositories. If you want to install software from a third-party software repository, you must add it to the package manager's list of available repositories.

Click <guibutton>Add</guibutton> to add a new repository.

Enter the APT line for the extra repository. This should be available from the website of the repository, and should look similar to the following:

Click <guibutton>Add Source</guibutton> and then click <guibutton>Close</guibutton> to save your changes.

You will be notified that the information about available software is out-of-date. Click <guibutton>Reload</guibutton>.

Packages from the new repository should now be available in your package manager.

As a security measure, most software repositories use a GPG key to digitally sign the files they provide. This makes it easy to check that the files have not been tampered with since their creation. In order for your package manager to be able to check this, you need the public key that corresponds to the signatures. The key should be available for download on the repository's website.